Generally, a vehicle is provided with a passenger protection device such as an airbag device and a seatbelt device for protecting a passenger in a collision of the vehicle or the like. In this case, for example, an airbag can be deployed in the collision to prevent the passenger sitting on the seat from colliding with an instrument panel of the vehicle and the like. That is, the airbag is to be deployed only in the case where there is a passenger on the seat. In the travel of the vehicle, it is possible that there is no passenger at seats (e.g., assistant seat) other than a driver seat where a driver is seated. Therefore, it is preferable for a passenger (sitting) detection system to be provided for the vehicle so as to actuate the airbag (e.g., airbag for assistant seat) only in the case where there is a passenger on the seat (e.g., assistant seat).
In this case, a capacitive sensor is often used in the passenger detection system, for example, referring to JP-11-271463. As shown in FIG. 8, a first electrode 100 (high-potential electrode) is arranged at an upper surface of an electrical-insulating film member (not shown). A second electrode (low-potential electrode) is constructed of a sitting frame 102 (of seat) which is made of a metal and connected with a vehicle chassis GND 108. There exists a capacitance Co between the first electrode 100 and the sitting frame 102.
A driving unit 104 which is arranged between the vehicle chassis GND 108 and the first electrode 100 applies a high-frequency voltage so that a weak electric field is formed. The lower half portion of the passenger sitting on the seat is positioned in the electric field so that a capacitance Cb of the electric field will vary responding to a sitting and a non-sitting of the seat. Thus, the sitting of the seat is detected by measuring the variation via an ammeter 106.
However, the seat may be watered due to rainwater entering the vehicle through an opened window or door of the vehicle, or drinking water which is spilled, or the like. The above-described passenger detection device has not been provided with measures against the watering of the seat. In this case, water (e.g., rainwater) entering the vehicle may seep into the seat through a surface member thereof, to reach the first electrode 100 (passenger detection electrode). Because the relative permittivity of water is larger than that of air, the gap between the lower half portion of the passenger and the passenger detection electrode will be changed when water reaches the passenger detection electrode, to have a same effect with the case where the area of the passenger detection electrode is enlarged. Thus, the capacitance Cb between the passenger detection electrode and the vehicle chassis GND is changed by water which enters the vehicle, so that it will be determined that there is a sitting on the seat in spite of no-sitting thereon.
Referring to JP-2002-350250A, a pressure-sensitive senor is mounted at the seat, considering the influence of watering of the seat. However, as compared with a capacitive sensor (used in the present invention) where there is a great influence due to entering water, there is not so much influence on the pressure-sensitive senor due to entering water.